* Extract custom tests from tests/flow/
Approach:
1. Remove all .js files in tests/flow except .snap.js files.
2. Copy over all .js files from tests/ in the flow repo.
3. Go through the diff looking for deletions.
- It was easy to see which deletions were due to changes in the tests
due to updates in the flow repo.
- For the rest of the deletions, I used `git blame` to verify that
they had been added by us since the flow tests were copied over.
This makes tests/flow/ simply a copy of the tests from the flow repo,
making it easier to sync with the upstream flow tests in the future.
* Add a script for syncing the flow tests
* Sync the flow tests
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